Line in
 

Is it possible, whether with Winamp, or some other player, to get the input from the sound card's line in connection to play through the player?

Yep.

Make sure you have your external source ready & playing. Open Winamp5, go to the File Menu & click on Play URL.

Then type in linein://

Close, but... Visualizations and stuff work, but the sound is the same. I'm trying to use a dsp and it doesn't seem to have any effect on the sound.

Thanks for the help!

Use Jasper Gronde's Line Input plugin
Uses the line:// command
See documentation on Jasper's site for instructions etc.

http://home.hccnet.nl/th.v.d.gronde/dev/index.html